Using induced charge detection to quantify the net charge of microdroplets within a quadrupole electrodynamic trap

Area of Science:

• Analytical Chemistry
• Physical Chemistry
• Electrochemistry

Background:

• Accurate microdroplet charge measurement is crucial for understanding charge's role in chemical reactions, ion distribution, and interfacial dynamics.
• Existing contactless charge measurement techniques lack the sensitivity or ability to measure charge on micron-sized particles, creating a significant research gap.

Purpose of the Study:

• To present a novel technique for directly measuring the net charge of microdroplets exiting a quadrupole electrodynamic trap (QET).
• To enable precise charge quantification on microdroplets, facilitating studies on charge-driven physical and chemical processes.

Main Methods:

• Utilized induced charge detection, where microdroplets induce charge (Qinduced) on a cylindrical electrode.
• Employed a homebuilt charge sensitive pre-amplifier (CSP) with a long time constant (1.02 ± 0.01 s⁻¹) for accurate detection of slow-moving microdroplets.
• Validated the method by comparing induced charge measurements with Faraday cup measurements (QFaraday cup) for approximately 2900 microdroplets.

Main Results:

• The induced charge detection method demonstrated high accuracy, with absolute differences averaging <5 fC (1% accuracy) when compared to Faraday cup measurements.
• Qinduced closely correlated with QFaraday cup across a range of droplet charges, sizes, and velocities.
• The technique proved effective for microdroplets interacting with the detection electrode for extended periods (up to 100s of ms).

Conclusions:

• The developed induced charge detection system provides a sensitive and accurate method for measuring microdroplet net charge.
• This technique addresses the limitations of existing methods and can be adapted for various droplet-based experiments, including droplet train studies.
• The system will advance research into how charge influences microdroplet physical and chemical processing, potentially revealing charge-accelerated chemistry mechanisms.